Environmental stressors are capable of causing genetic mutations in plants that result in severe damage to the photosynthetic apparatus. Light in the ultra-violet (UV) range is capable of causing the formation of thymidine dimers that result in genetic mutation in all organisms and in plants, severe damage to the photosynthetic apparatus. It is also the case that irradiation of plants with UV-B and the more energetic parts of UV-A causes major perturbations in cellular homeostasis and significant amounts of substrates, especially phenylalanine (Phe), are diverted from normal primary and secondary metabolic pathways into the production of specific UV screening pigments via the phenylpropanoid pathway. Ultra-violet-B (UV-B) radiation, for example, causes damage to plant life worldwide, and many mechanisms have evolved to help defend against the UV-A (320-400 nm) and UV-B (290-320 nm) that penetrates the atmosphere (Rozema et al., P. Photochem. Photobiol. B: Biology 66:2-12, 2002). Important to plant defenses are UV-screening compounds, such as the flavonoids, particularly inducible flavonols like quercetin (Rozema et al., P. Photochem. Photobiol. B: Biology 66:2-12, 2002; Stapleton and Walbot, Plant Physiology 105:881-889, 1994). Etiolated Arabidopsis seedlings, lacking a functional PD1 gene are unable to synthesize phenylalanine (Phe) and as a consequence, phenylpropanoid pigments (Warpeha et al., Plant Physiol. 140:844-855, 29006). Low doses of high energy UV-C (254 nm) are lethal to etiolated pd1 mutants (Phe absorbs up to 280 nm), gcr1 mutants and gpa1 mutants, but not to etiolated wt seedlings or seedlings of mutants of other members of the prephenate dehydratase (PD) family, indicating that this specific G-protein signaling pathway is critical to provide protection from UV via synthesis of Phe.
The erosion of the ozone layer and the consequential increase in UV exposure is an issue of increasing concern, as all populations are dependent upon vegetation for food, medicines and building materials. In addition to environmental stressors such as UV exposure, a number of biological stressors (e.g., fungi, bacteria, arthropods, etc.) cause harm to plants and threaten food crop yields. With the escalation of atmospheric and ecological onslaughts on plants and the continued increase in the world's population, there is a need to understand the pathways involved in conferring resistance to stressors and producing plants, especially crops, which are resistant to such onslaughts.